In common polymerization methods monomers are combined in the reaction vessel and allowed to react at random. Unless there is only one way that the polymer can form, sequences of repeating monomeric units will form as polymerization proceeds and the copolymer will be random. Random sequences are expected whenever a polymer is formed from two different monomers that have the same functional group. An example of such a random copolymer is a copolyester made from two monomers which have a hydroxy and carboxyl functionality such as copolymers of hydroxy benzoic acid and hydroxy napthoic acid, or a copolyimide in which each monomer has an amine and a carboxyl functionality. Another example is where more than two different monomers are combined, e.g., when two diols and one diacid form a polyester.
Ordered sequences are ordinarily expected only in copolymers made from a single monomer, or from two monomers that have different functionalities, e.g., when a diol and a diacid form a polyester.
Controlled sequences are known to occur in biological processes where specific complex molecules act as templates or sequenced controllers, e.g., DNA or RNA control in the growth of proteins.